The present invention relates to image display device having electron beam guides, and more particularly to a means for shielding the electron beams, as the beams enter the beam guides, from perturbing electric fields.
Recently, flat image display devices have been suggested utilizing a plurality of electron beam guides to direct electron beams to various positions on a cathodoluminescent screen. One type of these devices is described in U.S. Pat. No. 4,088,920 issued May 9, 1978 to W. W. Siekanowicz et al. entitled "Flat Display Device with Beam Guides." The electron beam guides in these devices comprises apertured plates which are biased so as to establish an electric field balance between two of the adjacent parallel plates. At the midpoint between the two adjacent plates, the electric fields are symmetrical so that an electron beam injected between the two plates will be confined by the balanced electric fields and guided in a path parallel to each of the plates. The Siekanowicz et al. patent also discloses that the display device may be provided with additional grids between the beam guides and the screen. These additional grids serve as focusing and accelerating grids for the electron beams as the beams flow from the beam guide to the phosphor screen. The potentials on the focus grid, acceleration grid, and screen electrode generate electric fields which can penetrate into the beam generating cathode region and perturb the electric fields around the cathode causing nonuniform injection of the electron beams into the beam guide. The electric fields generated by the modulation electrodes, the focusing and accelerating grids, and the screen electrode can also penetrate into the beam guide and perturb the fields in the critical transition region which extends within and along the beam guide for a longitudinal distance equal to at least the first transverse row of apertures. It is highly desirable in such display devices to shield the cathode region and the transition region from the perturbing electric fields emanating from the ends of the display device elements which cause nonuniform electron beam injection into the beam guides and also distort the balanced fields within the guides.